The chromophores that most readily fit this requirement are those with conjugated pi-

[duet062010]

The goal of a sunscreen chemical is simple—to prevent harmful UVB (and in some cases UVA) radiation

from penetrating the skin. There are two main ways in which this goal can be accomplished—by reflecting

(physically blocking) ultraviolet light or by absorbing UV light. Sunscreen chemicals are therefore put into

two groups based on which method they employ; they are classified as either physical blockers or chemical absorbers. Physical blockers, the most common of which is titanium dioxide, scatter all radiation in the UVB and UVA range. Titanium dioxide reflects light waves in the 290–770 nm range. However, the vast majority of commercial sunscreens are chemical absorbers.

Chemical absorbing sunscreens work on the principle of photo-excitation of electrons. They absorb photons of light of specific wavelengths and use the energy to promote electrons between energy levels. When the electrons later return to the ground energy state, they emit light at longer wavelengths (lower energies). Chemical species that exhibit this behavior are called chromophores. The specific wavelength absorbed by a given chromophore is determined by the discrete quantal amounts of energy that are required to excite electrons between the energy levels or its molecules. Since the primary objective of an absorbing sunscreen is to absorb UVB light (290–320 nm), the best sunscreens are those that absorb most heavily in this range. The chromophores that most readily fit this requirement are those with conjugated pi-bonding systems.

 

171. Based on the passage, which of the following can be inferred about the chromophores referred to in the final

sentence of the passage?

(A) If exposed to light with wavelengths of approximately 300 nm, they will scatter the radiation.

(B) If exposed to light with wavelengths in the 290–320 nm range, they will lower the energy level of

some of their constituent electrons.

© If exposed to light waves in the 290–770 nm range, they will absorb the photons and emit them as

light of longer wavelengths.

(D) If exposed to light with wavelengths of approximately 300 nm, some electrons in their component

molecules will switch to higher energy levels.

(E) If exposed to light waves in the 290–320 nm range, they will promote the discrete quantal amounts of

energy that are required to excite electrons between energy levels.

 

 

Here the answer is D. I am wondering why not E. Could someone please explain it to me?

Thanks in advance!

[beetest]

hello duet062010

 

first of all, thank you for posting this question. i am happy to see someone having same questions like me

its been more then one year you have posted this question so I am not sure if you still need to answer

but please let me share my views

answer E sayis that they will 'promote' the discrete quantal amounts of energy whereas it is the 'electrons' that get 'promoted' , not the quantals of energy.

in other words,

quantal amounts of energy - described as- required to excite electrons

electrons - described as - get promoted to next level

this might because i know abt the electron energy orbits in atom (my mother is chemistry teacher!)

but this very confusing question will help me realise tactic to focus on words in case of other non-familiar topics

my problem is that i dont have time to type put the entire paragraphs and questions as they are very long, i cudnt find them on internet to copy paste.

so I hope to find someone having the same book im reading, that is the Barrons 12th edition

I am not sure if i should expect a quick reply here as you might not be into this anymore, but if you can guide me to get my answers it would be realy great help

thanks in advance

beetest.